EP0519772B1 - Microwave antenna with optoelectronical controlled scanning - Google Patents

Description

The present invention relates to a microwave antenna using, for the pointing of its beam, a network of elementary reflectors with active elements capable of modifying, at will, under the control of an optical command, the length of the path of penetration of the microwave waves in the network reflectors to generate phase shifts varying from one elementary reflector to another and ensure the pointing of the antenna beam.

A known antenna of this kind has a reflector produced from a substrate made of a dielectric material with low losses for microwave waves, transparent to light, such as silica oxide SiO₂ or crystallized alumina AL₂ O₃ . On the side exposed to microwave frequencies, this substrate is coated with photoconductive elements insulated from one another by an electrically insulating material, possibly covered with an opaque layer transparent to microwave frequencies, and arranged in a network with a mesh pitch equal to λ / 2 to avoid multiple angles of reflection, λ being the wavelength of the microwaves considered. On the opposite side, not exposed to microwave frequencies, it is coated with a light-transparent electrode made of an electrically conductive material such as ITO tin oxide.

The photoconductive elements, which can be made of "intrinsic" silicon, that is to say insulating, are lit or not on command through the substrate and the transparent electrode, for example using a plated liquid crystal screen. against the substrate on the transparent electrode and lit by a light source. When lit, they become electrically conductive and reflect microwave waves before they enter the substrate. When they are not lit, they are electrically insulating and pass through the microwave waves which penetrate through the substrate and are reflected on the transparent electrode. If the propagation delay through the thicknesses of the photoconductive elements and of the substrate is close to an odd number of quarter periods of the microwave wave, the phase shift obtained between microwave waves depending on whether they meet an illuminated photoconductive element or a unlit photoconductive element is ¶.

One thus realizes a network of elementary reflectors, with a mesh with the step of half of the wavelength of the microwaves, which are each capable of generating at will phase shifts of 0 or ¶ under the control of an optical command. However, the performance of a scanning microwave antenna concerning the gain, the level of secondary lobes and the diffuse generally requires, to be acceptable, the use of a phase shifter controllable with more than two phase states at the level of each elementary reflector.

To meet this requirement, it has been proposed to stack layers of photoconductive silicon and low loss dielectric substrate in front of the transparent conductive electrode in order to present to the microwave wave, within each elementary reflector, different paths of staggered lengths. which correspond to various phase shift values between 0 and 2¶ and which are a function of the depth in the stack of the first photoconductive silicon layer made conductive by illumination. Difficulties are then encountered in the selective illumination of the different photoconductive silicon layers which mask each other.

The present invention aims to remedy these difficulties and allow obtaining phase controllers controllable in more than two phase states in a network of reflectors for microwave waves while retaining for the array of reflectors a simple three-layer structure formed of a substrate of dielectric material with low loss transparent to light carrying, on the side exposed to microwave frequencies, an array of photoconductive elements and, on the opposite side, a conductive electrode transparent to the light.

It relates to an optoelectronic scanning microwave antenna provided on the one hand with an array of elementary reflectors with optically controlled phase shifters comprising a substrate made of a dielectric material with low microwave losses, transparent to light, coated on the exposed side at microwave frequencies, a layer of photoconductive elements distributed in a network, and, on the opposite side, a conductive electrode transparent to light, and on the other hand, means for selective illumination of the photoconductive elements capable of passing the latter from an electrically insulating state to a conductive state and vice versa. This antenna is remarkable in that the network of photoconductive elements has a mesh which oversamples the mesh of the network of elementary reflectors. Thus, each elementary reflector groups together n² photoconductive elements, n being the oversampling rate, a greater or lesser proportion of which is illuminated, which gives it different phase states staggered from a minimum value obtained when all of its photoconductive elements are lit up to a maximum value obtained when all of its photoconductive elements are in the dark.

• une figure 1 représente, de manière schématique et partiellement démontée, une antenne hyperfréquence à balayage optoélectronique selon l'invention ;
• une figure 2 est un diagramme qui représente les variations du coefficient de réflexion sous incidence normale et du déphasage à la réflexion, en fonction de la résistivité, pour du silicium utilisé comme photoconducteur ;
• une figure 3 est un diagramme qui représente les variations du déphasage à la transmission et à la réflexion du silicium en fonction de la fréquence et
• une figure 4 illustre un exemple de répartition d'éléments photoconducteurs à la surface d'un réflecteur élémentaire de l'antenne représentée à la figure 1.

Other characteristics and advantages of the invention will emerge from the description of an embodiment given by way of example. This description will be made below, with reference to the drawing in which:

• Figure 1 shows, schematically and partially disassembled, an optoelectronic scanning microwave antenna according to the invention;
• FIG. 2 is a diagram which represents the variations of the reflection coefficient under normal incidence and phase shift to reflection, as a function of the resistivity, for silicon used as photoconductor;
• FIG. 3 is a diagram which represents the variations of the phase shift during transmission and at reflection of the silicon as a function of the frequency and
• FIG. 4 illustrates an example of distribution of photoconductive elements on the surface of an elementary reflector of the antenna shown in FIG. 1.

The microwave antenna shown in Figure 1 operates around 94 GHz. It comprises a horn 1 which illuminates with a microwave wave a plane array 2 of elementary reflectors placed in front of a liquid crystal screen 3 lit by a light source 4 through a focusing optic 5.

The network of elementary reflectors is in the form of a flat disc about 10 cm in diameter. It consists of a substrate 20 made of a dielectric material with low microwave losses, transparent to light, such as Silica oxide Si 0₂ or crystallized alumina AL₂ 0₃. On the side facing the horn 1, which is exposed to microwave frequencies, this substrate 20 carries a layer 21 of photoconductive elements such as silicon or gallium arsenide which are isolated from one another and distributed over the surface of the substrate to oversample the mesh of a network of elementary reflectors with the step of λ / 2 here, approximately 1.5 mm. On the side opposite the horn 1, the substrate 20 is coated with a conductive electrode 22 transparent to light which is, for example, made of tin oxide.

The liquid crystal screen 3 is pressed against the conductive electrode 22 of the substrate 20. It comprises an array of pixels which faithfully reproduce the distribution of the photoconductive elements 21 carried by the substrate 20 and which can be made, on command, either transparent , or opaque in order to selectively cause the lighting of the photoconductive elements placed in their extension.

The light source 4 can be a network of light-emitting diodes or of lasers providing a power of 30 to 50 Watts continuously at a wavelength of about 0.8 "m. The light intensity arriving on a silicon photoconductive element, when the pixel of the liquid crystal screen associated therewith is transparent, is then sufficient to make said element conductive.

FIG. 2 represents the variations of the reflection coefficient under normal incidence and of the phase shift at reflection, as a function of the resistivity, for silicon used as photoconductor. It shows that it is possible to pass from a total reflection to an almost total transmission of microwave waves with silicon whose resistivity varies from about 0.1 ohm.cm to more than 1000 ohm.cm depending on its illumination. It also shows that there is a lighting condition for which the silicon completely absorbs the microwave frequencies. This effect can be used to make the antenna absorbent, therefore stealthy for a detection system.

FIG. 3 represents the frequency response of the phase shift to the transmission (P = 1000 ohm.cm) and to the reflection (P = 0.18 ohm.cm) of the silicon. It shows that the phase shift during transmission is practically zero for a 94 GHz microwave wave.

FIG. 4 gives an example of distribution of the photoconductive elements on the surface of the substrate 20. These oversample the mesh of the network of elementary reflectors at the pitch of λ / 2 represented in solid lines, with a mesh four times finer represented in dotted lines . Thus, each elementary reflector is formed by a paving of 16 photoconductive elements 1a; ..., 4d which it is possible to illuminate individually via the pixels of the liquid crystal screen in order to make them at will. insulators or conductors. We can then choose a variable shape of the illuminated photoconductive surface in each elementary reflector to define a variable phase. This amounts to the introduction, into a microwave waveguide materialized by the contour of an elementary reflector, of a conductive iris which is equivalent to a susceptance for which we know how to calculate the phase in reflection. This variable susceptance can be the same for several microwave polarizations if they see equivalent surfaces.

For example, a horizontal polarization and a vertical polarization see the same phase shift if the photoconductive surface made conductive has a shape preserved in a rotation of ¶ / 2.

• une première configuration où aucun élément photoconducteur n'est éclairé ;
• une deuxième configuration, qui est celle représentée, où seuls les éléments photoconducteurs en coin 1a, 4a, 4d et 1d sont éclairés ;
• une troisième configuration où les éléments photoconducteurs 2a, 4b, 3d et 1c sont éclairés en plus des éléments photoconducteurs en coin 1a, 4a, 4d, et 1d ;
• une quatrième configuration où tous les éléments photoconducteurs du pourtour 1a, 2a, 3a, 4a, 4b, 4d, 3d, 2d, 1d, 1c, et 1b sont éclairés ;
• une cinquième configuration où tous les éléments photoconducteurs sont éclairés.

In the case illustrated by FIG. 4 where an elementary reflector consists of a paving of 16 photoconductive elements 1a, ..., 4d, five different configurations can be adopted, preserved by a rotation of ¶ / 2:

• a first configuration where no photoconductive element is lit;
• a second configuration, which is that shown, where only the corner photoconductive elements 1a, 4a, 4d and 1d are lit;
• a third configuration where the photoconductive elements 2a, 4b, 3d and 1c are lit in addition to the corner photoconductive elements 1a, 4a, 4d, and 1d;
• a fourth configuration where all the photoconductive elements of the periphery 1a, 2a, 3a, 4a, 4b, 4d, 3d, 2d, 1d, 1c, and 1b are lit;
• a fifth configuration where all the photoconductive elements are illuminated.

If the thicknesses of the photoconductive elements and of the substrate are of the order of half of the wavelength of the microwaves used, a two-bit controlled phase shifter independent of the polarization is obtained with the last four configurations.

Obviously, we can adopt a lower oversampling rate, for example two or three, but we then has a lesser choice of configurations or higher but we then come up against production difficulties due to the smallness of the photoconductive elements and of the pixels of the liquid crystal screen which must correspond to them.

Claims (3)

1. Optoelectronic scanning microwave antenna provided, on the one hand, with an array (2) of optically-controlled elementary reflectors comprising a substrate (20) made of a dielectric material having low loss at microwave frequencies and being transparent to light, the said substrate being coated, on the side exposed to the microwaves, with a layer (21) of photoconductive elements distributed in an array and, on the opposite side, with a conductive electrode (22) transparent to light and, on the other hand, with means (3, 4, 5) for selective illumination of the photoconductive elements, capable of making these elements pass from an electrically insulating state to a conductive state and vice versa, characterized in that the array of photoconductive elements forms a lattice which subdivides the lattice of the array of elementary reflectors.
2. Antenna according to Claim 1, characterized in that the array of photoconductive elements forms a lattice which subdivides the lattice of the array of elementary reflectors by a subdivision factor equal to four, each elementary reflector containing a tiling arrangement of sixteen photoconductive elements.
3. Antenna according to Claim 1, characterized in that the photoconductive elements belonging to an elementary reflector are illuminated selectively so as to form configurations of conductive elements and electrically insulating elements which are invariant under rotations π/2.

Images